Intragranular Type Research Articles

Investigation on intragranular and intergranular void growth and their competition in polycrystalline materials

In polycrystalline materials, void growth is strongly dependent on its surrounding microstructural environment such as grain morphologies, crystallographic orientations and void-grain boundary (GB) relative position. In this study, two kinds of void-GB relative positions are considered to investigate the orientation effect on void growth in FCC polycrystals: intragranular type, i.e., void located in interior of a grain; and intergranular type, i.e., void located at boundary of two adjacent grains. The analysis of orientation effect in intragranular (resp. intergranular) cases is conducted through a comparative study with single-crystal (resp. bi-crystal) counterparts. Moreover, the competition between the intragranular and intergranular void growth behaviors is statistically investigated for the first time. To this end, the representative volume element (RVE) of polycrystalline aggregates containing either intragranular or intergranular void is created by 3D Voronoi tessellation. Multiple realizations with different grain morphologies and crystallographic orientation permutations are implemented using crystal plasticity finite element (CPFE) simulations. We show that the mechanism for orientation effect on void growth in single crystals (resp. bi-crystals) is not preserved in intragranular (resp. intergranular) cases. According to the statistical analysis, both the intragranular and intergranular void growth follows a Gaussian distribution, whose profile is affected by both macroscopic stress triaxiality and Lode parameter. It is revealed that the statistical void growth distribution induced by random grain morphologies and crystallographic orientations is closely associated with the void-GB relative position (intragranular or intergranular). The present study may provide much richer insights into the mechanism of damage evolution in polycrystalline metals.

Effect of Abnormal Grains on the Mechanical Properties of FGH96 Solid-State Diffusion Bonding Joint

The hollow twin-web disk is designed to improve the thrust-to-weight ratio of the aero engine, where the welding joint microstructures determine the disk’s mechanical properties. This study aimed to elucidate the effect of abnormal grains formed in the welding region on the mechanical properties of FGH96 solid-state diffusion bonding joints. Digital image correlation using images captured by scanning electron microscopy (SEM-DIC) and electron backscattering diffraction (EBSD) technologies were applied. The results show that abnormally large grains (2.5 times that of the matrix), with preferred orientation in the bonding region, were detrimental to the joint mechanical properties. The yield and tensile strengths were 995.85 MPa and 1456.67 MPa, respectively, and the corresponding relative (ratio to the matrix) ones were 92.54% and 88.81%. After modifying the bonding process, the grain size in the bonding region was tailored to close to that of the matrix, and considerable twin boundaries (TBs) formed, leading to the relative tensile and yield strength reaching 98.86% and 99.37%. Furthermore, the failure mode changed to intragranular type from intergranular type. It demonstrates that tailoring the newborn grain size, introducing TBs inside, and eliminating preferred orientation during the welding process can be an efficient way to improve the joint mechanical properties.

Influence of spark plasma sintering temperature on the microstructure and strengthening mechanisms of discontinuous three-dimensional graphene-like network reinforced Cu matrix composites

For metal matrix composites (MMCs), the sintering temperature is of vital importance in realizing a dense bulk material with tight interfacial adhesion as well as limited structural damage to reinforcements. In this work, we investigated the influence of the spark plasma sintering (SPS) sintering temperature on the microstructure and mechanical properties of the in-situ synthesized discontinuous three-dimensional graphene-like network (3D GN) reinforced copper matrix composites. It demonstrates that with the rise of the sintering temperature from 600 °C to 700 °C, the GN distribution changes from intergranular type to intragranular type as a result of the significant improvement in interfacial wettability. Furthermore, the overall mechanical performance (both tensile strength and ductility) of the GN/Cu composites presents an unanticipated declining trend at elevated sintering temperatures. Integrated with microstructure characterization and the theoretical analysis, it is verified that the change of the reinforcement size and its distribution are the dominant factors. Their contributions to the mechanical properties of the composites were also studied in detail.

Tension-compression asymmetry in plasticity of nanotwinned 3C-SiC nanocrystals

Encompassing nanoscale thin twins in metals induces diverse influences, either strengthening triggered by the lattice dislocation blockage effects or softening prompted by dislocation emission from coherent twin boundary (CTB)/grain boundary (GB) intersections as well as CTB migration; yet the deformation mechanism remains poorly understood in ceramic nanostructures possessing covalent bonds. Here, we report the results of uniaxial compressive and tensile stress loading of twin-free and nanotwinned nanocrystalline cubic silicon carbide (3C-SiC) ceramic attained by large-scale molecular dynamics simulations. We find a strong and unique tension-compression asymmetry in strength of nanocrystalline ceramics, much higher than that of metals. We demystify that strength and ductility behaviour do not correlate simply with the amount of dislocation density, voids, intergranular disordered phase, and total strain energy; instead, it arises from a complex interplay of the aforementioned features and structural characteristics, e.g., GB and triple junction area distribution along/normal to the direction of straining as well as the capability of strain accommodation by the GBs and CTBs, with the dominant role of the structural characteristics in nanotwinned samples. Our results also reveal that primarily intergranular crack propagation and fracture along the GBs transpires, and not along the CTBs, resulting from the high energy of the GBs. However, a high density of nanoscale twins in the 3C-SiC nanocrystals could give rise to the alternation of the crack path from intergranular to intragranular type induced by shear, which brings about the glide of Shockley partials along the CTBs and subsequent formation of CTB steps and twin plane migration.

Statistical fatigue strength evaluation and inelastic deformation generated during static and cyclic loadings in Ce-TZP/alumina nanocomposite: part 2 – in water environment

The statistical fatigue strength evaluations in water of an intragranular type Ce-TZP/Al2O3 nanocomposite (Ce-TZP/A-N) were investigated in comparison to 3Y-TZP. The isothermal t-m transformation which causes low temperature aging degradation (LTAD) did not occur during fatigue tests for both materials. The strength degradations under static and cyclic loadings of Ce-TZP were fairly small compared to those of 3Y-TZP. The life shortening in water of 3Y-TZP was slight, and Ce-TZP/A-N maintained comparable fatigue lives found in air. Large inelastic deformations (converted strain=0.06–0.23%) were observed in the non-failure fatigued specimens after static and cyclic loadings. The amount of inelastic deformations and relevant transformed monoclinic contents in water were smaller than those in air at equivalent stress level. In contrast, the volume fractions of the monoclinic phase in water were slightly higher than those in air at equivalent strain level. For 3Y-TZP, no inelastic deformation was identified.

Statistical fatigue strength evaluation and inelastic deformation generated during static and cyclic loading in Ce-TZP/alumina nanocomposite: Part 1—in air environment

The statistical fatigue strength evaluations of an intragranular type Ce-TZP/Al2O3 nanocomposite (Ce-TZP/A-N), such as its initial strength, static and cyclic fatigue lives, and its dispersion, were investigated in comparison to 3Y-TZP. The strength degradations during static and cyclic loading of Ce-TZP/A-N were fairly small, and the dispersions of the fatigue life were also quite small compared to those of 3Y-TZP, especially for the case of cyclic loading. In addition, fairly large inelastic deformations (converted strain ≈0.1–0.3%) were observed in the non-failure fatigued specimens after both static and cyclic loading. The amount of inelastic deformations was generally higher under the static loading than under cyclic loading, and increased with increasing the applied stress. In contrast, no inelastic deformation was identified for 3Y-TZP. By means of X-ray diffraction analysis, a good correlation between the amount of inelastic strain and the transformed monoclinic content was recognized for both static and cyclic loading.

Influence of rolling conditions and aging process on mechanical properties of high manganese steels

Three laboratory cast and rolled high manganese steels with different Mn (in the range of 24–28wt%), C (in the interval of 0.2–1.2wt%) and Al (from 2.3wt% to 12wt%) contents has been investigated. Differences in chemical compositions, rolling conditions and followed by aging process at the temperature of 500°C and four different aging dwells (in the range of 6min to 60min) have led to various k-carbides precipitation. The k-carbides have been responsible for the mechanical properties. The intragranular type of nano-size has increased the strength, respectively hardness level whereas the k-carbides nucleated in the intergranular form have reached size of micrometre and in lower volume fraction have not been importantly able to increase the matrix hardness, especially in case of lower C portion. Balanced Mn, C and Al contents for optimised k-carbides precipitation have been also discussed. The materials were hot rolled to strips of 1.9mm in thickness from the heating temperature of 1100°C using the rolling mill Tandem.

Processing and properties of ZrB 2–SiC composites obtained by aqueous tape casting and hot pressing

ZrB 2–SiC composites with different SiC content were prepared through aqueous tape casting and hot pressing. The influences of dispersant, SiC content and binder content on the rheological properties of slurries were investigated and the conditions for preparing stable ZrB 2–SiC suspensions were optimized. After tape casting and drying, the green ZrB 2–SiC tapes showed good flexibility, lubricious surface and homogeneous microstructure. The ZrB 2 ceramics could be densified to 97.2% after hot-pressing, while the ZrB 2 containing 20 and 30 vol% SiC ceramics were nearly fully densified (>99%). The sintered ZrB 2–20 vol% SiC ceramic had improved mechanical properties compared with ZrB 2 ceramic. Further increase in SiC content resulted in lower flexural strength and fracture toughness. SEM and TEM showed a fine microstructure with a clear grain boundary. The fracture mode changed from intragranular type for ZrB 2 to both intragranular and intergranular type for ZrB 2–SiC composites.

Synthesis of Fe Based Metallic Glass–Pd Based Metallic Glass Composite by Slightly Pressured Liquid Phase Sintering

To consolidate [(Fe0.5Co0.5)0.75Si0.05B0.2]96Nb4 metallic glass powder to full density, a pressurized liquid phase sintering was employed, which was intended to promote densification by an enhanced wetting of a liquid phase with solid particles. Pd42.5Ni7.5P20Cu30 metallic glass powder, which has been reported to have a high glass forming ability and the lowest critical cooling rate of glass formation of 0.067 K/s, was chosen as a liquid phase component. The melting point of the Pd42.5Ni7.5P20Cu30 metallic glass alloy of 763 K is lower than the glass transition temperature of the [(Fe0.5Co0.5)0.75Si0.05B0.2]96Nb4 metallic glass alloy of 808 K. However, the wettability of this alloy with the Pd42.5Ni7.5P20Cu30 alloy was revealed to be poor. Therefore, sintering pressure was applied to the compacts to promote a viscous flow deformation of solid particles and also to enhance the intergranular permeation of the liquid phase. A specially designed micro-hot press was devised for the pressure-sintering experiment. A pseudo-wettability was observed during pressure-sintering, and the liquid phase was found to fill completely the intergranular space of the powder compact. The relative density of 64–95%, as well as the sintering structure, could be controlled by the punch displacement to squeeze out a part of the liquid phase from the compact. The compressive fracture strength of the obtained metallic glass composite was found to be as high as 2051 MPa, and the fracture was observed to be intragranular type, suggesting a good bonding in the particle-binding phase interface.

圧力付加液相焼結法による Fe 系金属ガラス-Pd 系金属ガラス複合材の作製

To consolidate [(Fe0.5Co0.5)0.75Si0.05B0.2]96Nb4 metallic glass powder to full density, a pressurized liquid phase sintering was employed, which was intended to promote densification by an enhanced wetting of a liquid phase with solid particles. Pd42.5Ni7.5P20Cu30 metallic glass powder, which has been reported to have a high glass forming ability and the lowest critical cooling rate of glass formation of 0.067 K/s, was chosen a liquid phase component. The melting point of the Pd42.5Ni7.5P20Cu30 metallic glass alloy of 763 K is lower than the glass transition temperature of the [(Fe0.5Co0.5)0.75Si0.05B0.2]96Nb4 metallic glass alloy of 808 K. However, the wettability of this alloy with the Pd42.5Ni7.5P20Cu30 alloy was revealed to be poor. Therefore, sintering pressure was applied to the compacts to promote a viscous flow deformation of solid particles and also to enhance the intergranular permeation of the liquid phase. A specially designed micro-hot press was devised for the pressure-sintering experiment. A pseudo wettability was observed during pressure-sintering, and the liquid phase was found to fill completely the intergranular space of the powder compact. The relative density of 64-95%, as well as the sintering structure, could be controlled by the punch displacement to squeeze out a part of the liquid phase from the compact. The compressive fracture strength of the obtained metallic glass composite was found to be as high as 2051 MPa, and the fracture was observed to be intragranular type, suggesting a good bonding in the particle-binding phase interface.

Prediction of fracture strength in Al2O3/SiCp ceramic matrix nanocomposites

Based primarily on a recent publication [S.M. Choi, H. Awaji, Sci. Tech. Adv. Mater. 6 (2005) 2–10.], where the dislocations around the nano-sized particles in the intra-granular type of ceramic matrix nanocomposites (CMNCs) were modeled, dislocation activities in Al2O3/SiCp CMNCs were discussed in relation to the processing conditions. The dislocations around the nano-sized particles, caused by the thermal mismatch between the ceramic matrix and nano-sized particles, were assumed to hold out the effect of Orowan-like strengthening, although the conventional Owowan loops induced by the movement of dislocations were unlikely in the ceramic matrix at room temperature. A model involving the yield strength of metal matrix nanocomposites (MMNCs), where the Owowan strengthening effect was taken into consideration, was thus modified and extended to predict the fracture strength of the intra-granular type of CMNCs without and with annealing. On the basis of the characteristics of dislocations in the CMNCs, the load-bearing effect and Orowan-like strengthening were considered before annealing, while the load-bearing effect and enhanced dislocation density strengthening were taken into account after annealing. The model prediction was found to be in agreement with the experimental data of Al2O3/SiCp nanocomposites reported in the literature.

Effect of Post Heating on Toughness of Bond Region for Duplex Stainless Steel Weldment

Post weld heating was applied to the simulated samples of the bond region of super duplex stainless steel in order to improve the toughness of weld bond region. The heating temperatures used were 1173K, 1273K and 1373K. The morphology of the secondary austenite changed from small intragranular type to massive austenite type as the heating temperature increased. The rate of austenite formation was fastest when the heating temperature was 1273K due to the C-curve kinetics. When the heating time exceeded 120s, the austenite content reached 50% in any heating temperatures. Charpy impact tests were performed using the specimens in which austenite content reached 50%. In the case of the heating temperature of 1173K, the toughness was hardly improved probably because of the precipitation of chromium nitrides or small amount of sigma phase. When the heating temperature was 1273K, good toughness comparable to the base metal could be obtained. It was found that the massive intragranular austenite arrested the cleavage crack and formed small steps and tear ridges among the cleavage facets from the fractographic observation based on three-dimensional morphology and observation of the cross sectional microstructure of cleavage fracture surface.

Preparation of TiC ceramics through aqueous tape casting

TiC laminates are prepared through aqueous tape casting and hot pressing technique. Commercial TiC powders were firstly treated and the surface charge was determined through zeta potential measurement. The stability of concentrated TiC suspensions was characterized by rheological test. Results showed that the isoelectric point of as treated TiC particles moved slight toward more acid region, while the zeta potential curves decrease significantly. TiC suspensions exhibit a high degree of particle stabilization. After tape casting, TiC sheets are laminated and hot pressed. The strength and fracture toughness of TiC samples at 1850 °C are 532 MPa and 6.41 MPa m 1/2, respectively. The fracture mode is a mixture of intergranular and intragranular type.

Fabrication of Al<SUB>2</SUB>O<SUB>3</SUB>/ZrO<SUB>2</SUB> Micro/Nano-Composite Prepared by High Energy Ball Milling

An Al 2 O 3 -5 vol%ZrO 2 micro/nano-composite was successfully fabricated by sintering the high-energy ball milled mixture of commercia Al 2 O 3 powder and zirconium alkoxide, The composite could be sintered to a nearly full density at 1450°C in the ambient atmosphere. Th microstructure consisted of the fine dispersions of ZrO 2 particles, both intra-granular type of ∼ 50 nm and inter-granular type of ∼ 0.2 μm, an Al 2 O 3 matrix grains of ∼ 0.6μm. The role of high energy ball milling in the present nano-composite formation was primarily the refinemen of the Al 2 O 3 grains and the introduction of strains in them.

Tough and strong Ce-TZP/Alumina nanocomposites doped with titania

To develop a new attractive Ce-TZP ceramic, which possesses a high strength while still preserving significant high toughness, we investigated an intragranular type of nanocomposite in lower CeO 2 content for 0–1 mol% TiO 2 doped Ce-TZP/Al 2O 3 system. These composites partly possessed an intragranular microstructure, in which several of 10–100 nm sized Al 2O 3 particles were trapped within the ZrO 2 grains. Furthermore, elongated Al 2O 3-like phases were produced at the ZrO 2 grain boundaries, which were in-situ precipitated during the sintering process. TiO 2 was confirmed to dissolve into the tetragonal ZrO 2 lattice, which was determined to be effective for strengthening with a slight addition due to its grain growth enhancing ability on ZrO 2. For an optimum component with 0.05 mol% TiO 2 doped 10Ce-TZP/30 vol% Al 2O 3 composite, both high strength (950 MPa) and high toughness (18.3 MPa.m 1/2 for the IF method, 9.8 MPa.m 1/2 for the SEVNB method) were achieved thus breaking through the strength–toughness tradeoff relation in transformation toughened ZrO 2 and its composite materials.

The effect of TiO 2 addition on strengthening and toughening in intragranular type of 12Ce-TZP/Al 2O 3 nanocomposites

To compensate the definite disadvantages of lower strength and lower hardness in Ce-TZP, an intragranular type of 12Ce-TZP/0–30 vol% Al 2O 3 nanocomposite was investigated. Furthermore, the effect of TiO 2 addition on strengthening and toughening was also examined for the optimum component of 30 vol% Al 2O 3, which showed a maximum strength of 866 MPa. For these composites, a novel interpenetrated intragranular microstructure was presented, in which either 200-nm-sized Al 2O 3 particles or 30–50-nm-sized ZrO 2 particles were trapped within the ZrO 2 grains or Al 2O 3 grains, respectively. The TiO 2 was confirmed to dissolve into the tetragonal ZrO 2 lattice, and contributed to the phase stability of the tetragonal phase and the promotion of intragranular nano-dispersion. For the small amount (0.2 mol%) of TiO 2 doped 12Ce-TZP/30 vol% Al 2O 3 composite, simultaneous improvements in strength (1012 MPa) and toughness ( 10.2 MPa · m 1 2 for the IF method, 5.7 MPa · m 1 2 for the SEVNB method) were achieved through tailoring the sintering condition.

Low-intensity reciprocity failure of a AgBr emulsion: effect of halogen acceptors.

Low-intensity reciprocity failure (LIRF) of a silver bromide emulsion was drastically reduced, but not eliminated, by the addition of a halogen acceptor, such as acetone semicarbazone, before coating. The remaining failure has a slope of approximately minus unity on an isodensity plot of log intensity vs log intensity × time. This is interpreted as exemplifying two types of LIRF, an intergranular type stopped by a sufficient amount of halogen acceptor, and an intragranular type caused by the thermal instability of a photolytically produced silver atom. A LIRF curve was also obtained in terms of the number of absorbed photons necessary to make a grain developable. These data suggest that considerable recombination occurs before nucleation of the latent image. The quantum efficiency of formation of print-out silver is 0.2 silver atom per absorbed photon. This value indicates that growth of the latent image after nucleation is efficient.